Conventionally, the patterned etching of thin metallic films is accomplished by the use of photolithographic masks. For example, an etch-resistant coating ("a resist") is laid down upon a semiconductor wafer. A pattern is created by further coating portions of the resist with an opaque material and then irradiating the material to break down the exposed resist. The exposed resist is then removed by developing and the wafer may then be etched to produce a patterned film. This example illustrates positive resists, wherein the pattern which remains after development corresponds to the opaque regions. Negative resists are also known in the art.
The mask for the masking process normally comprises a layer of photosensitive resist material formed on the surface of a protective SiO.sub.2 layer on a substrate. Typical photo-resist materials comprise organic materials which undergo chemical changes, such as molecular cross-linking, when subjected to UV radiation. The photo-resist layer is illuminated with ultraviolet light passed through a photo mask containing the desired pattern to be formed in the photo-resist layer. The photo-resist layer is developed to expose selected portions on the SiO.sub.2 layer and the exposed portions are dissolved by acid solutions. Next, the remaining resist material i stripped away and a dopant is introduced to the semiconductor through the unprotected portions of the pattern.
The foregoing process is subject to a number of deficiencies, especially when attempting to use this process for submicrometer patterning of thin films where extremely high resolution is required. Some of the problems associated with such conventional masking systems are the following:
The organic photoresist materials are not inert to acids nor resistant to plasma treatments creating difficulties in maintaining the integrity of the mask pattern. Generally, the organic resist material will not survive temperatures above 200.degree. C., which may occur in subsequent processing of the semiconductor wafer. Such resists also contain carbon and require organic solvents which may cause contamination unless extreme precaution is used.
Organic light sensitive resists must usually be formed in layer thicknesses of about 1 micron for satisfactory operation. The resolution of a mask is inversely proportional to mask thickness. The thinner the mask, the better the resolution. To achieve better resolution, thin X-ray sensitive masks of organic material have been utilized, but these masks require relatively long exposure time.
Organic resist materials are spun onto the surface, thus requiring the introduction of spinning equipment into the process with attendant problems of contamination.
The ability to etch thin films without using a mask would be an important processing capability for both the microelectronics and the photovoltaics industry. Elimination of the necessity of masking during thin film patterning would reduce the complexity and number of steps in, say, the etching step of Integrated Circuit (IC) chip formation. Further, it would make it more economical to produce custom or one-of-a-kind designs, since the expensive mask production step would be eliminated. One method for maskless etching is described in U.S. Pat. No. 4,619,894. In this method, an aluminum oxide layer on the substrate is patterned by selective heating. Conventional etching methods (such as acid bath immersion) are then used to expose the pattern.
Another method for producing patterned surfaces without using masks is described in U.S. Pat. No. 4,615,904. In this method, a thin absorption layer is formed upon a substrate surface, portions of the absorption layer are prenucleated using a focused energy beam, and deposition is induced with deposition occurring primarily upon prenucleated sections of the substrate. This method suffers from the inability to entirely prevent deposition upon non-prenucleated surfaces, resulting in deposition in areas where it is undesirable.
A third method of maskless etching (described in U.S. Pat. No. 4,622,095) involves exposing a metallized substrate to a beam of laser radiation in a halogen gas atmosphere to form a metal halide salt reaction product on the substrate. This metal halide salt is then removed from the substrate using a solvent for the metal halide salt.
In yet another method of maskless etching (described in U.S. Pat. No. 4,490,210) a metallized substrate is exposed to a selected gas which spontaneously reacts with the metal to form a solid reaction product with the metal by a partial consumption of the metal. A radiation beam of a wavelength suitable for absorption by the reaction product and/or the metal is applied in a desired pattern to vaporize the reaction product and selectively etch the metal.
The need still exists, however, for methods and apparatus for producing etched, patterned films without masks or liquid etchants and which does not adversely affect metal regions not subject to etching.